Using the new vtkQWidgetWidget class in VTK (event handling). Which in turn uses a vtkQWidgetRepresentation which uses a vtkQWidgetTexture (the actual drawing). The basic gist is we share a texture between Qt and VTK. Qt paints the texture, VTK then renders it as part of the VR world.
This MR has the key pieces https://gitlab.kitware.com/vtk/vtk/merge_requests/5553/diffs